EP-A-0 566 266 discloses an injection mold for manufacturing disc-shaped plastic objects with a central hole, which mold comprises:
a first mold part;
a second mold part displaceable relative to this first mold part by means of first displacing means;
which two mold parts are displaceable between a closed first position, in which they together partially bound a mold cavity corresponding to the shape of an object for manufacture, and an open second position in which a formed object can be removed;
a third mold part which in the closed position of the first and second mold parts extends through the mold cavity defined thereby and in the region of this mold cavity has a shape corresponding to the shape of the central hole, which third mold part is axially displaceable between a first position, in which it partially forms a boundary of the mold cavity, and a retracted second position by means of second displacing means by means of a guide sleeve forming part of the first mold part;
which third mold part bounds a channel which is connectable on the free outer end to the injection nozzle of an injection molding device and on the other side debouches with an injection inlet in to the region of the mold cavity in the first position of the first and the second mold parts and the first position of the third mold part, and in the second position of the third mold part debouches blind against the inner wall of the guide sleeve; and
heating means for keeping plastic present in the channel in the plastic state.
It is an object of the invention to improve the prior art injection mold in a number of respects. The invention provides an injection mold for manufacturing disc-shaped plastic objects with a central hole, which mold comprises:
a first mold part;
a second mold part displaceable relative to the first mold part by means of first displacing means;
which two mold parts are displaceable by means of the first displacing means between a closed first position, in which they together partially bound a mold cavity corresponding to the shape of an object for manufacture, and an open second position in which a formed object can be removed;
a third mold part which in an active position extends in the closed position of the first and second mold parts through the mold cavity defined thereby and in the region of this mold cavity has a shape corresponding to the shape of the central hole, which third mold part is axially displaceable relative to the first and second mold parts between said active position in which it forms a partial boundary of the mold cavity and a retracted rest position by means of second displacing means by means of a guide sleeve forming part of the first mold part;
which third mold part bounds with the guide sleeve a channel which is connectable at its free end remote from the mold cavity to the injection nozzle of an injection molding device,
which channel can be selectively opened and closed at its end adjacent to the mold cavity by means of a valve consisting of a valve seat which is formed by a part of the inner surface of the guide sleeve widening towards the mold cavity and a valve body which is formed by a widened portion on the relevant end of the third mold part such that in the open active situation of the valve the channel debouches in annular manner into the mold cavity and in the closed rest position of the valve the channel is separated from the mold cavity;
which valve body can be displaced to the active position of the valve by the pressure of the plastic supplied through the channel and can be displaced to the rest position by a pressure member extending through the second mold part, wherein the activity of the pressure member and the supply of plastic never occur simultaneously; and
first heating means co-acting with the guide sleeve for keeping plastic present in the channel in the plastic state.
In order to combine a reliable closure of the valve with a low pressure on valve body and valve seat in the contact surfaces, an embodiment is recommended in which the valve body has substantially the same shape as the valve seat.
Use can be made of an annular ring of passage openings with which the channel debouches in the mold cavity. Preferably, the channel debouches in the mold cavity via an annular opening. In this preferred embodiment an inflow is ensured which is as homogenous as possible.
In a specific embodiment at least the outer surface of the valve body has a hardness which corresponds at least to that of hard metal. For example, at least the outer surface may consist of ceramic material.
In a specific embodiment the third mold part is carried slidably by the guide sleeve by means of a number of guide protrusions present on the guide sleeve and/or on the third mold part. The guide protrusions can be arranged as desired on the inner surface of the guide sleeve or on the outer surface of the third mold part, or a combination thereof. These guide protrusions can advantageously be placed in angularly equidistant manner. For reasons of production technique, it is recommended to form the guide protrusions, for instance in the form of ribs, on the third mold part. The guide protrusions must then have a certain dimension in an axial direction or use is made of separate protrusions located at mutual axial distance. A reliable axial guiding is hereby realized. The guide protrusions preferably have such a large active surface area in common with the surface co-acting therewith that the pressure between the relevant surfaces is very small. This enhances the lifespan and low friction.
A particular embodiment has the special feature that the third mold part is carried slidably by the guide sleeve by means of a number of guide ribs present on the third mold part which extend in angularly equidistant manner and each have a helical shape. The part-flows mutually separated by the guiding ribs form seams which connect poorly to each other and are distinguishable in the final manufactured product. The helical ribs have the effect that the part-flows mix well after being combined. This phenomenon, also known as xe2x80x9ccold flowxe2x80x9d is thus essentially prevented.
For optical information carriers such as CD-audio, CD-ROM, CD-R, CD-i, DVD and the like, it is of the greatest importance that the optical quality of the information carriers, usually consisting of polycarbonate, meets the very highest set standards. In order to realize this objective the mold according to the invention preferably comprises first cooling means acting in the center zone of the mold cavity for cooling plastic flowing into the mold cavity, which center zone extends over an area with a diameter of (0.30 0.15) times the diameter of the mold cavity. A substantial quality improvement can be realized by thus tempering the inflowing plastic.
According to another aspect of the invention, when the valve body has the same shape as the valve seat and the channel debouches via an annular opening into the mold cavity, the mold according to the invention can have a conical shape. In one embodiment, the top angle of the conical shape has a value of (20xc2x15)xc2x0.
According to another aspect of the invention, the pressure member is a pressure cylinder which is provided with second cooling means.
In order to allow the above described guide protrusions to move with the least possible friction over the surface co-acting therewith, the guide protrusions may co-act with the adjacent surface by means of a friction-reducing coating, for instance TiN, DLC (diamond-like carbon). Such a coating must be heat-resistant and wear-resistant.
According to yet another aspect of the invention, the coating is applied to all surfaces of the channel and has anti-stick properties relative to the plastic flow-past, for instance TiN, DLC (diamond-like carbon).
In a specific embodiment, the valve body is releasably connected to the rest of the third mold part and consists of a material with poor heat conduction, for instance a ceramic material. In this case, the valve body, which is subject to erosion and ageing by pressure, can be replaced as desired. The low thermal conduction through the valve body also prevents formation of undesired thermal couplings.
In order to keep the operation of a mold as manageable as possible, the heating means are separated from the first cooling means. Specifically, an annular thermal insulator may be arranged between the guide sleeve and the first cooling means.
The invention further provides an injection molding unit comprising at least two injection molds for manufacturing disc-shaped plastic objects with a central hole, each of which molds comprises:
a first mold part;
a second mold part displaceable relative to this first mold part by means of first displacing means;
which two mold parts are displaceable by means of the first displacing means between a closed first position, in which they together partially bound a mold cavity corresponding to the shape of an object for manufacture, and an open second position in which a formed object can be removed;
a third mold part which in an active position extends in the closed position of the first and second mold parts through the mold cavity defined thereby and in the region of this mold cavity has a shape corresponding to the shape of the central hole, which third mold part is axially displaceable relative to the first and the second mold part between said active position in which it forms a partial boundary of the mold cavity and a retracted rest position by means of second displacing means by means of a guide sleeve forming part of the first mold part; which third mold part together with the guide sleeve bounds a channel which is connectable at its free end remote from the mold cavity to the injection nozzle of an injection molding device,
which channel can be selectively opened and closed at its end adjacent to the mold cavity by means of a valve consisting of a valve seat which is formed by a part of the inner surface of the guide sleeve widening towards the mold cavity and a valve body which is formed by a widened portion on the relevant end of the third mold part such that in the open active situation of the valve the channel debouches in annular manner into the mold cavity and in the closed rest position of the valve the channel is separated from the mold cavity;
which valve body can be displaced to the active position of the valve by the pressure of the plastic supplied through the channel and can be displaced to the rest position by a pressure member extending through the second mold. part, wherein the activity of the pressure member and the supply of plastic never occur simultaneously; and
first heating means co-acting with the guide sleeve for keeping plastic present in the channel in plastic state.